[libc++][docs] Major update to the documentation

- Landing page: add link to the libc++ Discord channel
- Landing page: reorder "Getting Involved" above "Design documents"
- Landing page: remove "Notes and Known Issues" which was completely outdated
- Rename "Using Libc++" to "User Documentation" and update contents
- Rename "Building Libc++" to "Vendor Documentation" and update contents

The "BuildingLibcxx" and "UsingLibcxx" pages have basically been used for
vendor and user documentation respectively. However, they were named in
a way that doesn't really make that clear. Renaming the pages now gives
us a location to clearly document what we target at vendors and what we
target at users, and to do that separately.

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